1. Field of the Invention
The present invention relates to information input devices and display devices, and particularly to an information input device and a display device including a panel provided with a capacitive touch sensor for detecting the position to which an object to be sensed is brought close.
2. Description of the Related Art
Display devices such as liquid crystal display devices and organic EL display devices have advantages such as small thickness, light weight, and low power consumption. Therefore, these display devices are frequently used in mobile-use electronic apparatus such as cellular phones and digital cameras.
As one kind of such display devices, the liquid crystal display device has a liquid crystal panel formed by enclosing a liquid crystal layer between a pair of substrates as a display panel. The liquid crystal panel is e.g. a transmissive panel. Specifically, illuminating light emitted from an illuminating unit such as a backlight provided on the back side of the liquid crystal panel is modulated by the liquid crystal panel and passes through the liquid crystal panel. By the modulated illuminating light, image displaying is carried out on the front side of the liquid crystal panel.
This liquid crystal panel is based on e.g. the active-matrix system and includes a TFT array substrate over which plural thin film transistors (TFTs) functioning as pixel switching elements are formed. Furthermore, in this liquid crystal panel, a counter substrate is so disposed as to be opposed to this TFT array substrate and the liquid crystal layer is provided between the TFT array substrate and the counter substrate. In this liquid crystal panel of the active-matrix system, the pixel switching element inputs potential to a pixel electrode to thereby apply voltage to the liquid crystal layer and control the transmittance of the light passing through the pixel. Thereby, the image displaying is carried out.
For the above-described display device, a touch panel is often provided as an information input device on the display panel in order to allow the user to input operation data by utilizing images such as icons displayed on the screen of the display panel.
In addition to the display device in which the touch panel is provided as an external unit on the display panel, a display device in which the display panel has a built-in touch panel function has also been proposed.
For example, display panels provided with a capacitive touch sensor have been proposed (refer to e.g. Japanese Patent Laid-open Nos. 2008-9750, 2009-3916, and 2008-129708).
In these display panels, the capacitive touch sensor is so configured that the electrostatic capacitance changes when an object to be sensed (also referred to as a sensing object) is brought close to the sensing surface, and the position at which the sensing object is brought close to the sensing surface is detected based on the change in the electrostatic capacitance.
FIGS. 30A and 30B are diagrams showing the appearance when a capacitive touch sensor TS is driven. FIG. 30A shows the case in which a sensing object F is not brought close to the sensing surface of the touch sensor TS. On the other hand, FIG. 30B shows the case in which the sensing object F is brought close to the sensing surface.
As shown in FIGS. 30A and 30B, for the capacitive touch sensor TS, for example a pair of electrodes, i.e. a scanning electrode 23J and a detecting electrode 24J, are opposed to each other with the intermediary of a dielectric substance Y, so that the capacitive element is formed.
If the sensing object F is not brought close to the sensing surface, an electric field is generated between the scanning electrode 23J and the detecting electrode 24J as shown in FIG. 30A when a common potential Vcom is applied to the scanning electrode 23J, which serves as the drive electrode.
On the other hand, if the sensing object F having high electrostatic capacitance, such as a finger, is brought close to the sensing surface, as shown in FIG. 30B, the fringe electric field (the dotted-line part in the diagram) is blocked by the sensing object F.
Therefore, the electrostatic capacitance based on the scanning electrode 23J and the detecting electrode 24J varies depending on whether or not the sensing object F is present. Thus, based on the change in the electrostatic capacitance, the position at which the sensing object F is brought close to the sensing surface is detected.
However, in the case of the above-described capacitive touch sensor, its detection sensitivity is often not sufficiently high and thus it is often difficult to detect the touch position with high accuracy.
For example, if the electrostatic capacitance based on the scanning electrode and the detecting electrode is significantly lower than the parasitic capacitance of the detector, the detection is often not favorable, which causes the need to increase the width of the detecting electrode. However, in this case, the fringe electric field is blocked by this wide detecting electrode, and thus the lowering of the detection sensitivity often Occurs.
Furthermore, if the detecting electrode is formed as a transparent electrode composed of ITO or the like, an attempt to ensure higher transparency of the detecting electrode raises the resistivity of the detecting electrode and thus causes increase in the time constant. This often results in a long detection time.
As above, the touch sensor often involves insufficient detection sensitivity and a long detection time, and therefore it is often difficult for the touch sensor to perform the detection with high accuracy.
There is a need for the present invention to provide a display device and an information input device that can easily realize detection with high accuracy.